1. The Field of the Invention
This invention relates to screwdrivers and more particularly to that class of screwdriving tools utilized with tamper proof screw heads wherein such screw heads are adapted to be driven in a clockwise direction utilizing a conventional screwdriving blade and further adapted to remain in such driven condition without being able to be removed by further use of a conventional screwdriver blade when operated in the opposite direction.
2. Description of the Prior Art
The prior art abounds with screwdrivers and screwdriving blades useful for driving a variety of screw heads. U.S. Pat. No. 3,658,105 issued to John Burt on Apr. 25, 1972 teaches a fastener driving arrangement in which there is a recess in the head of the fastener, having outwardly extending pockets with parallel sidewalls and bottomwalls defined by spherical segments, the driver having blades adapted to fit in the recess pocket, the blades having end walls that are complementary to the inner recess wall and side walls which are adjacent the inner bottom corner of each recess pocket and diverse progressively away from the recessed side wall in both the radial and axial direction so that, that when torque loads are applied, the recess wall will be caused to gradually increase the area of contact between the driver and the recess. This apparatus obviously provides a screwdriver blade for use with a special screw head which may be driven in both directions, utilizing a symmetrical pocket formed in the head of the blade wherein the symmetry is the same regardless of the direction of rotation of the screw head.
U.S. Pat. No. 3,695,321 issued Oct. 3, 1972 to J. W. Garehime, Jr. discloses the upper surface of the head of the screw having wedge shaped cavities arranged on either side of, or around, a central hub or land. The cavities have undercut sidewalls, and the land has a central hole, preferably circular in shape. The driving tool, such as a screwdriver, or the like, is provided with multiple blades which are spaced apart on either side of or around a central aperture. A central pin descends from the shank of the screwdriver and is adapted to engage the hole in the central land of the screw. The bottoms of the screwdriver blades are of expanded thickness so that when these blades are inserted in and engage the screw head cavities, the expanded bottom portions of the blades will lock under the undercut side walls of the cavity. Mating of the screwdriver blades in the screw head cavities is guided, thereby allowing stabilization by engagement of the screwdriver central pin in the hole in the central end of the screw head surface. Expandable pin sections maintained under tension may be employed to temporarily lock a screw on the screwdriver blade and hold in positive axial alignment with said screwdriver for insertion and withdrawal. Thus, the Garehime apparatus discloses a stabilized screwdriver whose screw engaging portion is caused to reside in touching engagement with surfaces which are symmetrically disposed, regardless of the direction of operation of the screwdriver.
U.S. Pat. No. 3,800,841 issued on Apr. 2, 1974 to F. L. Lindberg describes a screwdriver with a replaceable nib for use in production procedures to permit replacement of the nib as a wear part. The split nib has a distal end separation which in combination allows a resilient movement of the nibs to grasp the head of the screwdriver prior to insertion. Clearly, this invention discloses a useful tool whose life may be prolonged, by replacing the nib and which is effective in driving a screw in one direction and is useful in also removing the same screw. It may be stated, however, that the Lindberg apparatus is useful as a security tool by virtue of the unique shape of the convolutions and cavities formed in the screw head. In this sense, this apparatus is useful to permit unauthorized tampering of the screw.